1. Field of the Invention
The present invention relates to a developing unit, and an electrophotographic image forming apparatus including the developing unit.
2. Description of the Related Art
For example, in the conventional electrophotographic image forming apparatus, such as a dry type color laser printer, an electrostatic latent image is formed on a photosensitive medium, and the electrostatic latent image is developed with toner powder. The developed image is transferred to a sheet of print paper via a predetermined transfer mechanism. The conventional electrophotographic image forming apparatus is disclosed in Japanese Patent Publication No. Hei 8-334951, Japanese Patent Publication No. Hei 8-110710, Japanese Patent Publication No. Hei 2-275970, and Japanese Patent Publication No. Hei 10-186775.
FIG. 1 shows an example of a typical electrophotographic image forming apparatus. Referring to FIG. 1, the image forming apparatus includes a photoreceptive drum 10 which is a photosensitive medium, a charging unit 11 charging the photoreceptive drum 10; a laser scanning unit (LSU) 12 which is an exposing unit scanning light onto the charged photoreceptive drum 10 to form an electrostatic latent image, a developing unit 13 developing the electrostatic latent image using toners of four colors, that is, yellow (Y), magenta (M), cyan (C), and black (K), a transfer unit including a transfer belt 14 sequentially receiving four-color images developed on the photoreceptive drum 10 to overlap one another to produce a desired color image and transfer the color image on a sheet of paper, and a fusing unit 15 fixing the transferred color image by pressing and heating the paper. Four developing apparatuses Y, M, C, K (13-Y, 13-M, 13-C, and 13-K) provided in the developing unit 13 are all elastically biased by a predetermined spring (not shown) in a direction separated from the photoreceptive drum 10. When the developing apparatuses Y, M, C, K (13-Y, 13-M, 13-C, and 13-K) are selectively moved toward the photoreceptive drum 10 according to a rotation of a cam 13b, a developing roller 13a located at a leading end of each developing apparatus Y, M, C, K (13-Y, 13-M, 13-C, and 13-K) accesses the photoreceptive drum 10. The image forming apparatus further includes a paper cassette 16, a photoreceptive drum cleaning unit 17, and a discharging unit 18.
In the image forming apparatus having the above structure, an image forming process is performed as follows. First, when the photoreceptive drum 10 is charged by the charging unit 11, the LSU 12 scans the light to form the electrostatic latent image to be developed with first color toner. For example, when a yellow color is to be developed first, the yellow color developing apparatus 13-Y accesses the photoreceptive drum 10 and develops the electrostatic latent image formed on the photoreceptive drum 10 with yellow color toner. The developed yellow image is transferred to the transfer belt 14. Next, another electrostatic latent image for a second color is formed by charging and exposing of the photoreceptive drum 10. If the second color is magenta, the magenta color developing apparatus 13-M accesses the photoreceptive drum 10 and develops the another electrostatic latent image formed on the photoreceptive drum 10 with magenta color toner, and the developed magenta image is transferred to the transfer belt 14 where the yellow image is already transferred, to overlap the yellow image. Images of cyan, a third color, and black, a fourth color, are developed and transferred to the transfer belt 14 in the same manner so that the color image of a desired color is finally formed on the transfer belt 14. Then, the completed color image is transferred to the paper supplied between the transfer belt 14 and a transfer backup roller 14a. As passing through the fusing unit 15, the color image is completely fixed to the paper by being heated and pressed.
Here, the respective developing apparatuses Y, M, C, K (13-Y, 13-M, 13-C, and 13-K) of the developing unit 13 have a structure shown in FIG. 2. FIG. 2 shows the black developing apparatus 13-K of the four color developing apparatuses Y, M, C, K (13-Y, 13-M, 13-C, and 13-K) as an example. As shown in FIG. 2, the black developing apparatus 13-K includes a main body 13d slidably supported by a guide slot 19a of a frame 19, a developing roller 13a supplying toner contained in the main body 13d to a surface of the photoreceptive drum 10 having a gap g with the developing roller 13a to attach the toner to the surface of the photoreceptive drum 10, and a gap maintenance roller 13c installed coaxially with the developing roller 13a to maintain the development gap g. Thus, when the main body 13d of the developing apparatus is driven by a cam 13b to access the photoreceptive drum 10, the gap maintenance roller 13c contacts the photoreceptive drum 10 to form the development gap g. In this state, the toner adhering to the developing roller 13a is transferred to the photoreceptive drum 10 where the electrostatic latent image is formed, via the developing gap g by a difference in electrical potential.
However, in the above structure, since the four color developing apparatuses Y, M, C, K (13-Y, 13-M, 13-C, and 13-K) alternately access the photoreceptive drum 10 and retreat therefrom to form the color image, an impact generated by the gap maintenance roller 13c colliding against the photoreceptive drum 10 is continuously generated. Then, an error can be generated in the developed image formed on the photoreceptive drum 10. Thus, as shown in FIG. 3, a method has recently been suggested, in which the four developing apparatuses 13-Y, 13-M, 13-C, and 13-K are elastically biased by a spring 20, so that they can be fixedly disposed close to the photoreceptive drum 10, and the development process performed by a developing apparatus to obtain the desired color image can be selected by adjusting the difference in the electrical potential between the respective developing apparatuses 13-Y, 13-M, 13-C, and 13-K and the photoreceptive drum 10.
However, in the above methods in which the respective developing apparatuses 13-Y, 13-M, 13-C, and 13-K alternately accesses the photoreceptive drum 10 for development as shown in FIG. 1, and in which the gap maintenance rollers 13c of the respective developing apparatuses 13-Y, 13-M, 13-C, and 13-K closely contacts the photoreceptive drum 10 for development as shown in FIG. 3, a direction A in which the developing apparatuses 13-Y, 13-M, 13-C, and 13-K move simultaneously, includes a component (normal direction of a surface of the photoreceptive drum 10) G variable according to the development gap g and components (tangential or alignment directions of the surface of the photoreceptive drum 10) B, C, D, and E variable according to an alignment between the developing roller 13a and the photoreceptive drum 10. That is, as the gap maintenance roller 13c moves in the direction A, displacements are simultaneously generated in the development gap g in the normal direction G and the alignment directions B, C, D, and E. Here, the displacement in each of the alignment directions B, C, D, and E matters. The displacement in each of the alignment directions B, C, D, and E indicates an unbalanced state between a center axis of the development roller 13a and an axis of the photoreceptive drum 10. In this case, even when the gap maintenance roller 13c accurately contacts the photoreceptive drum 10, the development gap g is changed at both ends of the developing roller 13a and the center of the photoreceptive drum 10 so that an image having uniform concentration cannot be obtained. The above defect results from inconsistency between the direction A, in which the gap maintenance roller 13c moves, and the normal direction G of the development gap g. Therefore, a development unit having an improved structure to solve the above problem is needed.